1. Field of the Invention
This invention relates to a magnetic recording medium and more particularly a magnetic recording medium having excellent electromagnetic conversion characteristics, improved durability, and reduced dropouts.
2. Description of the Related Art
Various magnetic recording media have been proposed for the purpose of increasing-recording density and improving high frequency characteristics.
For example, Japanese Patent Application Laid-Open 4-113510 discloses a magnetic recording medium having a double magnetic layer formed by double coating in order to record low frequency signals and high frequency signals simultaneously. The upper magnetic layer of this magnetic recording medium has a coercive force of 1200 to 2000 Oe, and the lower magnetic layer comprises a Co-containing ferromagnetic oxide. The coercive force of the lower layer is not more than 80% of that of the upper layer. Japanese Patent Application Laid-Open 2-110824 describes a magnetic recording medium having a double-coated magnetic layer, in which the squareness ratio of the lower layer is smaller than that of the upper layer by at least 0.02. Additionally, a magnetic recording medium having a double-coated magnetic layer in which the upper layer has a thickness of 0.5 to 1.0 .mu.m while the lower layer has a thickness of 2.5 to 3.5 .mu.m (see Japanese Patent Application Laid-Open 62-231426) and a magnetic recording medium having a double-coated magnetic layer in which the upper layer to lower layer thickness ratio is 0.4 to 1.0, which were proposed for the same purpose as described above, are also known.
In order to obtain a high output in an attempt to realize a further increased recording density, Japanese Patent Application Laid-Open 6-4854 proposes a magnetic recording medium having a double-coated layer in which the lower layer comprises a nonmagnetic substance and the upper layer comprises a magnetic substance and has a thickness of not more than 1.0 .mu.m. However, such a magnetic recording medium cannot assure sufficient output characteristics in recording long wavelengths of 2 .mu.m or longer because the thickness of the upper layer is too small. It follows that the magnetic recording medium fails to obtain well-balanced output characteristics over a low-to-high frequency region.
Further, Japanese Patent Application Laid-Open 62-236132 discloses a magnetic recording medium having a double-coated magnetic layer in which an upper layer comprises vertically oriented barium ferrite and a lower layer comprises an acicular ferromagnetic metal oxide or ferromagnetic metal alloy. Because both the upper and lower layers comprise magnetic powder, the magnetic layers, as a whole, have a considerable thickness. As a result, appreciable influences of self-demagnetization are exerted, making it difficult to obtain a high output in the high frequency region as compared with the above-described thin film type magnetic recording media having a magnetic/nonmagnetic layer structure as disclosed in Japanese Patent Application Laid-Open 6-4854.